Synchrotron Xray Photoelectron Studies of the CCl4 Chemistry on

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Synchrotron X-ray Photoelectron Studies of the
CCl4 Chemistry on Fe3O4 (111)-2 ? 2 Surfaces
K. Adib 1, N. Camillone 2, J.P. Fitts 3, K.T. Rim
3, Z. Zhu 1, T. Müller 3, G.W. Flynn 3, S.A.
Joyce 4, D.R. Mullins 5, R.M. Osgood, Jr 2,3
1. Materials Science Program, Department of
Applied Physics and Applied Mathematics, Columbia
University, NY, NY 2. Brookhaven National
Laboratory, Upton, NY 3. Environmental Molecular
Sciences Institute, Columbia University, New
York, NY 4. Los Alamos National Laboratory, Los
Alamos, NM 5. Oak Ridge National Laboratory, Oak
Ridge, TN
48th American Vacuum Society Meeting, San
Francisco, 2001
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Rationale

• Iron oxides ideal MODEL minerals
• Abundant on the crust of the Earth
• Fe/O stoichiometry, multiple valences (Fe3,
  Fe2)
• Lattice spacings
• CCl4 well studied chemistry
• An actual soil contaminant

Our chemical probes
(1) Molecular level chemistry of the adsorbate
(CCl4) CCl4 dissociatively adsorbs on Fe3O4 (111)
surface producing adsorbed CCl2 and Cl fragments
XPS (2) Reactive sites of the substrate (
Fe3O4 (111)-2?2 ) Upon subsequent heating the
fragments can react with each other or abstract
substrate atoms both Fe and O sites are needed
- TPD
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Experimental Techniques
UHV LEED, TPD, XPS

• Natural Hematite crystal from Brazil
• Cut and polished into wafers (0001) orientation
• Contaminants K, Ca and Al, Na, Ti
• K 0.3

Example of hematite

• X-ray Photoelectron Spectroscopy
• National Synchrotron Light Source, BNL
• Core level spectra

• Temperature Programmed Desorption
• Columbia University
• Mass spectroscopic detection of thermal
  desorption of previously deposited CCl4

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Sample Preparation (0001) After Ar Sputter
As Dependent on Temperature and Oxygen Pressure
1000 K
1200 K
gt1273 K
Temp
Temp



After G. Ketteler, W. Weiss, W. Ranke, R.
Schlögl Phys. Chem. Chem. Phys. 3 (2001) 1114
After N.G. Condon, F.M. Leisble, A.R.
Lennie, P.W. Murray, T.M. Parker, D.J. Vaughan,
G. Thornton, Surf. Sci. 397 (1998) 278
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TPD Results Fe3O4 (111) - 2 ? 2
CCl4 dosed at 100 K

• Reaction products
• Recombinative CCl4
• OCCl2
• C2Cl4
• FeCl2
• But NOT C2Cl6, FeCl3
• CO2 not detected

• CCl2 is the reaction precursor

(1) CCl2 Olattice ? OCCl2 (2a) CCl2
2 .Cl ? CCl4 (2b) CCl2 CCl2 ? CCl4 C
(2c) .CCl3 .Cl ? CCl4 minor (3)
CCl2 CCl2 ? C2Cl4
Competing channels (not elementary reaction
steps)
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K. Adib, N. Camillone, et al. Surf. Sci., in press
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Fe3O4 (111) - 2 ?2 Competitive Reactions

• CCl4 desorption coincides with C2Cl4 desorption
• Desorption of both is suppressed when OCCl2
  desorbs

86
(1) CCl2 Olattice ? OCCl2 (2a) CCl2
2 .Cl ? CCl4 (2b) CCl2 CCl2 ? CCl4 C
(2c) .CCl3 .Cl ? CCl4 minor (3)
CCl2 CCl2 ? C2Cl4
11

• Competing channels
• (not elementary reaction steps)

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X-ray Photoelectron Spectroscopy
TPD by nature perturbs the chemistry. Need to
detect the species while still on the surface.

• U12A beamline, National Synchrotron Light Source,
  BNL
• UHV 1 ? 10-10 Torr
• Choice of Fe 2p, O 1s, Cl 2p, C 1s
• Resolution 0.4 0.7 eV

• Fe 2p too broad to show shifts
• C 1s, Cl 2p and O1s
• CCl2 on the surface?
• How is CCl2 chemically attached to oxygen?
• Nature of iron chloride species?

P. Liu, T. Kendelewicz, G.E. Brown Jr, E.J.
Nelson and S.A. Chambers, Surf. Sci. 417 (1998)
53 T. Kendelewicz, P. Liu, C.S. Doyle, G.E. Brown
Jr, E.J. Nelson and S.A. Chambers, Surf. Sci. 453
(2000) 32
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XPS C 1s, Cl 2p
Heating of Surface after Exposure consistent
with TPD
emphasis
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XPS Surface Species 100 K
Cl 2p
C 1s
K 2p3/2
2p1/2

• Cl 2p 4 species
• CCl4
• CCl2
• TWO iron-chloride species

• CCl2 is formed at 100 K
• C 1s separation 3.9 eV
• Consistent with CCl4 and CCl2 on pure Fe
• K 2p

J. Lara, H. Molero, A. Ramirez-Cuesta and W.T.
Tysoe, Langmuir 12 (1996) 2488
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XPS Oxygen species
CCl4 dosed surface
After long exposure
Undosed surface

• Lattice oxygen 530.0 0.1 eV Final state
  peak 531.1 eV OCCl2 peak expected 533-534 eV
• No oxygen core level shift
• Only after long exposure is spurious condensed
  H2O peak seen

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XPS Iron chloride species
After anneal to 600 K no longer associated with
carbon

• Two iron-chloride species
• Separation 1.1 eV not consistent with
  FeCl2/FeCl3 (0.2 eV)
• Shift not consistent with FeCl2/FeCl3

Not necessarily two types of iron sites
National Institute of Standard and Technology
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XPS Iron chloride species FeCl2 and FeCl
Effect of Annealing
900 K
300 K

• Lower binding energy feature converts to FeCl2
• Species may be FeCl2 and FeCl

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Summary of XPS results
CCl4 dissociatively adsorbs on Fe3O4 (111)-(2?2)
at 100K
Surface oxygen

• Upon subsequent heating of surface the adsorbed
  fragments can
• Recombine to CCl4 (11)
• Associatively recombine to C2Cl4 (3)
• Abstract surface oxygen to form OCCl2 (86)

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TPD Fe2O3/Fe1-xO BIPHASE Different Reactions
CCl4 dosed at 100 K

• Dissociative adsorption recombinative
  desorption
• No reaction with surface atoms no OCCl2, C2Cl4
  or iron chlorides
• No uncapped oxygen

Biphase


Fe3O4 (111)-2?2
CCl4 recombination Eact ? 0.7 eV
After G. Ketteler, W. Weiss, W. Ranke, R.
Schlögl Phys. Chem. Chem. Phys. 3 (2001) 1114
After N.G. Condon, F.M. Leisble, A.R.
Lennie, P.W. Murray, T.M. Parker, D.J. Vaughan,
G. Thornton, Surf. Sci. 397 (1998) 278
15
Schematic Summary of Reaction Sites

• At 100 K CCl4 dissociatively adsorbs on both
  Fe3O4 (111)-(2?2) and Fe2O3/Fe1-xO structured
  biphase
• Dissociation of CCl4 on Fe3O4 (111)-(2?2) results
  (at 100 K)
• Weakly bound CCl2 diradical
• Chemisorbed Cl

• However, both Lewis acid and Lewis base sites are
  needed for subsequent reactions with the surface
  atoms
• Iron chlorides consist of FeCl and FeCl2